Arbitrary ordering?

Question

Querying dba_constraints, I might finish with an 'order by constraint_type'. But this now means that unique constraints are listed after foreign keys because 'R' sorts before 'U'. If I'm running the query to generate the SQL to re-create the constraints, I'll get a lot of failures because the foreign keys are supposed to reference primary or uniquely-constrained columns, and the unique constraints won't have been created yet. It's no good me ordering by constraint_type desc, either, because then I'll be trying to create foreign keys before the primary keys get created, because 'R' would sort before 'P'. What I really need is all the primary constraints, then all the uniques and all the foreign keys bringing up the rear.
So my question: is there a way of ordering by a column in an arbitrary, user-defined way? Where you say, for example, "order by constraint_type ('P','U','C','R')".
A bit like list partitioning, where you get to define what "constrains" a record to a partitcular partition, only this time, you get to define your ordering sequence.
As another example, I want to sort the EMP table in department number order, but the ordering I want is "20, then 10, then 40 and then 30", for whatever weird business reason you might think up.
Do-able in a simple (or not-so simple) query? I can see how four separate union queries could do it... am wondering if there's anything more elegant and that could extend easily to lots more than 4 distinct ordering values.

Answer 1

Decode the constraint type and order by the decoded column.  In SQLPlus, you can even NOPRINT the ordering column.
SQL> create table hkc_test_90 (table_name varchar2(30), col_name varchar2(30), constraint_type varchar2(1));
Table created.
SQL> insert into hkc_test_90 values ('T_A','C_1','P');
1 row created.
SQL> insert into hkc_test_90 values ('T_A','C_3','U');
1 row created.
SQL> insert into hkc_test_90 values ('T_B','C_1','P');
1 row created.
SQL> insert into hkc_test_90 values ('T_B','C_2','R');
1 row created.
SQL> insert into hkc_test_90 values ('T_C','C_1','P');
1 row created.
SQL> insert into hkc_test_90 values ('T_C','C_4','R');
1 row created.
SQL> commit;
Commit complete.
SQL>
SQL> select table_name, col_name, constraint_type, decode(constraint_type,'P',1,'U',2,'C',3,'R',4) ctype_no
  2  from hkc_test_90
  3  order by ctype_no, table_name;
TABLE_NAME                     COL_NAME                       C   CTYPE_NO
T_A                            C_1                            P          1
T_B                            C_1                            P          1
T_C                            C_1                            P          1
T_A                            C_3                            U          2
T_B                            C_2                            R          4
T_C                            C_4                            R          4
6 rows selected.
SQL> col ctype_no noprint
SQL> select table_name, col_name, constraint_type, decode(constraint_type,'P',1,'U',2,'C',3,'R',4) ctype_no
  2  from hkc_test_90
  3  order by ctype_no, table_name;
TABLE_NAME                     COL_NAME                       C
T_A                            C_1                            P
T_B                            C_1                            P
T_C                            C_1                            P
T_A                            C_3                            U
T_B                            C_2                            R
T_C                            C_4                            R
6 rows selected.
SQL>
Hemant K Chitale